Conventionally, digital data such as audio data recorded on a recording medium which enables high-speed access such as CD (compact disc) can be copied at a high speed and easily. Moreover, duplicate data resulting from copying of digital data has little deterioration from original information. Therefore, in view of the copyright protection, it has been strongly demanded to take effective measures against unauthorized copying and various techniques have been proposed for preventing unauthorized copying.
For example, a method is proposed in which information for copy prevention is embedded in main data such as audio data in a format which does not affect the main data. According to this method, only a device which can reproduce the information for copy prevention is allowed to reproduce the main data, and therefore this method is effective for preventing unauthorized copying.
However, even if sub data such as information for copy prevention is embedded in main data in a format which does not affect the main data, as described above, it is difficult to prevent unauthorized copying when transmission (or copying) of data equivalent to data recorded on a disc itself, that is, bits of recorded data is carried out at a modulation data level.